McGwire, the St. Louis Cardinals slugger chasing the Roger Maris record for most home runs in a season, confirmed that he has been taking a daily dose of androstenedione, a precursor of the male hormone testosterone, for the past year.

Although it's legal in baseball, the supplement is banned in some other sports, including collegiate athletics, professional football and Olympic competitions.

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And although supplemental testosterone is a dangerous drug available only through prescriptions, substances of the type used by McGwire are perfectly legal to buy over the counter and are widely used by body builders.

Taking a hormone precursor like androstenedione (pronounced andro-STEEN-die-own) is not the same thing -- nor nearly as dangerous -- as taking a straight shot of testosterone itself.

Research suggests that only about 5 percent of the precursor is converted to the active hormone, and testosterone levels in the blood quickly return to normal levels. A newer supplement, called androstenediol, converts at a 15 percent rate.

Promoters bill these wares as "natural" nutritional aids that merely provide the building blocks for better performance.

But doctors view the supplements as weak relatives of synthetic anabolic steroids, powerful performance boosters with well- known dangerous side effects.

Weekend athletes hoping to pump themselves into shape McGwire-style probably should stick to their Wheaties, experts said yesterday, adding that children should be especially careful.

"For young kids coming up, playing with the amount of circulating hormones can have a much more profound effect than in an adult man," said Robert Dudley, senior vice president at Unimed Pharmaceuticals, a Chicago company developing a prescription-only testosterone gel for treating hormone deficiencies.

Bone abnormalities and short stature are among the potential side effects in a growing child, he said.

Another danger stems from the fact that the body self-regulates hormonal levels and may permanently reduce natural testosterone output in response to prolonged supplementation.

Steve Plisk, director of sports conditioning at Yale University, said he steers clear of recommending any form of hormone supplementation, partly because the substances are banned in so many sports.

"It's not necessarily blood doping, but it's a gray area," Plisk said. "If it's not illegal, it's close enough, and it could be banned at some point."

Some advocates, however, use the link to steroids as a selling point. In the body-building magazine Muscle Mag International, for example, writer Bruce Kneller recently said a stronger form of the same substance McGwire uses, called androstenediol, "is as close to being a true anabolic steroid as you are going to get and still remain legal to sell."

And the St. Louis Cardinals defended McGwire's use of the pills, calling androstenedione "a natural substance" which "has no proven anabolic steroid effect nor significant side effects," and which increases natural testosterone levels for only a period of about an hour.

In fact, Brian Batcheldor, a research chemist at a firm called Extreme Performance Consultants, which advises supplement makers, insisted that androstenedione is harmless. He said regular users report definite benefits, including increased workload capacity and faster recovery from exercise.

But other experts cast doubts on that reasoning, suggesting it could be simply a placebo effect: People who take something they believe works often perceive that it does.

A person would probably have to take far more than the recommended dosage of either androstenedione or androstenediol to cause themselves harm, doctors said. But since there is little or no hard scientific evidence, "We don't really know how well it works, or if it works" at all, according to Dr. William Ross, a sports-medicine specialist at St. Francis Medical Center in San Francisco.

If that's the case, then the biggest danger of copying McGwire's pill-popping habit may be financial. On the Internet, supplement makers yesterday were already using McGwire to promote their wares. Androstenedione was being marketed yesterday at $23 for 60 100-mg capsules